While Klopp and Guardiola exude positivity, Mourinho has cultivated a seemingly toxic environment at Old Trafford.

In recent months, the 55-year-old Portuguese has looked increasingly irritable and isolated.

United sit 19 points behind Liverpool, languishing in sixth place.

Over five years, the board has put in place a succession of lucrative commercial deals but it has faltered when it comes to recruitment of players and coaches.

It's notable that United are one of only four Premier League clubs without a sporting director. Its youth academy, meanwhile, no longer produces talent like the vaunted "Class of '92," a group of players made up of David Beckham, Gary Neville, Phil Neville and Paul Scholes who were the bedrock of the great Alex Ferguson's teams.

As for Mourinho, the swagger and charisma which made him one of the hottest properties in world football has gone.

The suave, sexy, "special" Mourinho -- who conquered Europe with Porto, transformed Chelsea into the dominant force in English football and dragged Inter Milan to Champions League glory -- has been replaced by a snarling, shadow of his former self.

David Moyes, Louis van Gaal and now Mourinho have all tried and failed to replicate the success enjoyed by the legendary Ferguson, who claimed 13 Premier League titles and two Champions League triumphs before ending his glorious 26-year reign at United in 2013.

The steel and verve which came to define Ferguson's teams has disappeared entirely, replaced by a turgid brand of football played by a disillusioned squad of players.

And Mourinho's relationship with that squad -- expensively assembled over recent years by the club's much-maligned CEO Ed Woodward -- has deteriorated to the point where players of unquestionable potential rarely perform to their abilities, while others aren't even given the opportunity.

Notably Paul Pogba, United's record signing when he arrived from Juventus in 2016, who watched Sunday's dreadful defeat to Liverpool from the substitutes' bench.

A star of France's vibrant 2018 World Cup-winning team, Pogba has been unable -- or perhaps unwilling -- to replicate that form for United.

The situation with Pogba perfectly encapsulates what United must rectify with their next appointment.

While Klopp has taken players to new heights -- see Mohamed Salah's incredible goalscoring numbers for the 2017-18 season -- Mourinho has dragged his down.

Given Mourinho's habit of publicly singling out players for criticism, the Old Trafford hierarchy will be keen to make sure their new coach's management style is more carrot than stick.

One man who ticks that box is Tottenham Hotspur's Mauricio Pochettino.

The Argentine has developed a Tottenham team that regularly competes with the Premier League's elite, built around a core of exciting young players like Harry Kane and Dele Alli.

Spurs consistently qualify for the Champions League despite lacking the financial firepower of teams like United, Manchester City, Liverpool and Chelsea.

It is unlikely, however, Pochettino would abandon what he's so carefully built over four years for the chaos at United.

His effervescent Spurs team is yet to convert its potential into tangible, trophy success. Pochettino will surely want to achieve at least that when the club moves into its spectacular new stadium in 2019.

Pochettino is one of many candidates who would be unlikely to vacate their current jobs for the United hot seat. But there is someone who is available.

Zinedine Zidane was untested in top-level management when he replaced Carlo Ancelotti at Real Madrid in 2016.

By the time he left the Spanish capital earlier this season, the Frenchman had overseen three consecutive Champions League triumphs to go with a league title in 2017.

His success at the world's biggest football club, combined with his availability, makes him the outstanding candidate to replace Mourinho.

Who better to get the best out of Pogba than a man who knows all about leading France to World Cup glory?

Zidane was the undoubted star of France's glorious team of 1998, while he also dazzled at Pogba's former club Juventus.

It's a safe bet that Zidane would have more luck getting a tune out of the midfielder than Mourinho.

The arrival of one of the greatest players to have ever laced up a pair of boots would undoubtedly inspire United's world-weary squad.

Attacking talents like Romelu Lukaku, Marcus Rashford, Anthony Martial and Alexis Sanchez have seemed at a loss this season. "Zizou" could change all that.

But there are doubts as to whether Zidane would be willing to end his current career break for a stint in Manchester.

Earlier this season, his agent Alain Migliaccio dismissed speculation Zidane could arrive at Old Trafford and suggested the 46-year-old has no desire to work in England.

Whether that is still the case now the job is available remains to be seen.

One thing is certain. United needed a coach of Zidane's caliber five years ago. They can't afford to miss out on him now.